A new species of Acerentulus Berlese, 1908 (Protura, Acerentomata, Acerentomidae) from Bulgaria with a revised key to the confinis group

Abstract A new species, Acerentulus bulgaricussp. nov., belonging to the confinis group, is described from Bulgaria. This species is characterized by long foretarsal sensilla a and b, the posterior position of foretarsal seta δ4, the presence of seta P1a on abdominal tergites II–VII and seta P3a on abdominal tergite VII, possession of eight anterior setae on abdominal tergite VII and composed spsm pores on sternite VI. The new species differs from all members of the confinis group in possessing P1a setae on tergites II–VII. Otherwise it is similar in body chaetotaxy and porotaxy to three species of the cunhai group, A. proximus, A. correzeanus and A. tuxeni. The identification key to 22 Acerentulus species belonging to confinis group is revised.


Introduction
The proturan genus Acerentulus Berlese, 1908 is widely distributed over the Holarctic, reaching Southern America, Australia and New Zealand. According to the Catalogue of the World Protura (Szeptycki 2007), the genus comprises 40 species and 2 subspecies. Acerentulus rapoporti Condé, 1963, which was noted by Szeptycki (2007) as "species incertae sedis", was recently synonymized with Andinentulus ebbei (Tuxen, 1984) (Shrubovych et al. 2014a). Several species, noted as "species inquirendae" in Szeptycki (2007), are not placeable; type materials were lost for Acerentulus americanus Hilton, 1943and A. shensiensis Chou & Yang, 1964, and A. aubertoti Condé, 1944 was described from a prelarva (see Tuxen 1955, Szeptycki 2007. Since Szeptycki (2007) additional species have been described, bringing the current total to 48 species (Wu and Yin 2007, 2014b, Galli and Capurro 2013, Galli et al. 2017. Distributions and taxonomic differentiation between the 21 species within the confinis group was discussed previously ). The present paper contains a description of a new Acerentulus species from Bulgaria, which belongs to the confinis group. Only three species have been recorded from Bulgaria till now: Acerentulus confinis (Berlese, 1908), A. gisini Condé, 1952and A. traegardhi Ionesco, 1937(Szeptycki 2007 contrary to well-studied neighboring countries: 11 and 8 Acerentulus species were recorded in Serbia and in Romania respectively (Blesić andMitrovski-Bogdanović 2012, Shrubovych andFiera 2016). An identification key to the confinis group of species worldwide is updated and reorganized according to morphological characters.

Material and methods
Protura specimens, collected in Bulgaria from 2015 to 2018, were extracted from soil samples with Berlese-Tullgren funnels into 95% ethanol. All specimens were mounted on glass slides in Faure's medium (Dunger and Fiedler 1989). Additional material was analyzed in the collection of J. Rusek, deposited in the Institute of Soil Biology, Biology Centre of the Czech Academy of Sciences (ISB).
The holotype and other materials of D. Georgiev and C. Fiera are deposited in the collection of the Institute of Systematics and Evolution of Animals, Krakow, Poland (ISEA). One female paratype (ISB A-791.1) and materials of J. Rusek are deposited in the collection of the Institute of Soil Biology, Biology Centre, Czech Academy of Sciences. One female paratype (SMNH 90.1) is deposited in the collection of the State Museum of Natural History, Lviv, Ukraine (SMNH).
Remarks. This species belongs to the confinis group of Acerentulus species characterized by long foretarsal sensilla a and b; only Acerentulus berruezanus Aldaba, 1983 is characterized in possessing P1a setae on tergites I and VII (setae P1a absent on tergites II-VI). Other members of the confinis group are characterized by the absence of P1a setae on tergites I-VI. Three species from the cunhai group, A. proximus Szeptycki, 1997, A. correseanus Szeptycki, 1997and A. tuxeni Rusek, 1966, have a similar chaetotaxy (setae P1a present on tergites II to VII, eight A-setae on tergite VII) and porotaxy (notal sterna and sternites I-III without pores, sternites IV-V with 1+1 spsm pores, sternite VII with a spm pore, sternite VI with two groups of spsm pores 2+2 or 2+3 in some males, except A. tuxeni, which has 1+1 spsm pores). However, as members of the cunhai group these three species have short sensilla a and b.
In preimago specimens the A4 setae on tergite VII are absent symmetrically or asymmetrically. Setae P1a on tergites II-VII and P3a on tergite VII appear in the maturus junior stage (Fig. 4A, B). All pores on the tergites and sternites are present by the maturus junior stage, except for the posterosublateral pores (psl) on tergite VII, which appear in the preimago.

Discussion
In a previous key to the confinis group ) the presence of six or eight anterior setae on tergite VI was used as the first character to divide the species. In the current study we found this character to be quite variable, with frequent asymmetrical absence of setae A4 and A2 on tergite VI. Szeptycki (1991) confirmed a high degree of variability of these characters in Acerentulus exiguus Condé, 1944, A. xerophilus Szeptycki, 1979, A. cunhai Condé, 1950, A. traegardhi Ionescu, 1937and A. tuxeni Rusek, 1966. Therefore, it can be difficult to decide how many setae are present on tergite VI. Seta A4 on tergite VI appears mostly in the adult stage (Aldaba 1984, Imadaté 1988, and this may be a reason for high variability of this character. Therefore, we have improved the identification key by using presumably more stable characters that appear in earlier stages of acerentomid postembryonic development, such as the presence of accessory setae P1a and P3a on tergites and seta on sternite XI in the maturus junior stage. According to Szeptycki (1991) sternal porotaxy is a good taxonomic character. Sternal pores are easily visible and practically identical with adult porotaxy from the maturus junior stage. Foretarsal sensillum shapes, proportional lengths, and location of sensilla and setae are also stable characters from larva II to adult (Shrubovych and Rusek 2010).
Key to the Acerentulus confinis species group (valid from maturus junior stage)